This invention relates to speed controllers for driving AC motors at variable speeds by the change of output frequency, and particularly to a moment-of-inertia detecting method and apparatus for automatically detecting a resultant moment of inertia of the AC motor and a load connected thereto, and a speed control method and apparatus for driving the an AC motor at the optimum acceleration or deceleration on the basis of the measured moment of inertia.
When an AC motor is driven at a variable speed by a speed controller, the torque upon change of speed is greatly changed by the moment of inertia of the load connected to the AC motor. Therefore, overcurrent may flow and the speed controller may be tripped unless the speed is changed in a manner allowing for the moment of inertia of the load.
A conventional method of computing moment of inertia is described in Japanese Patent Laid-open Gazette 61-88780. This method computes the moment of inertia from the integral of a torque proportional signal and the amount of change of revolution speed.
In this prior art, if the moment of inertia is found and the rate of change of speed is decided, operation without overcurrent and trip can be realized, but the speed of revolution must be detected. Thus, a speed detector is necessary. In addition, a is disclosed (see Japanese Patent Laid-open Gazette 59-156191). In this system, an induced electromotive force detector, generally not used, is necessary for the general-use inverter, thus causing the construction to be complicated and the cost to increase.